648 



CIRCULATION. 



der, and tail. There is, however, considerable 

 variety in regard to the distribution of the 

 posterior abdominal veins in fishes; and com- 

 parative anatomists do not appear as yet to 

 have connected these varieties with any general 

 view of their uses. In the Gadus the venous 

 blood from the tail and middle of the ab- 

 domen goes to the kidneys only by venae ad- 

 vehentes. In. the Silurus the blood of the 

 posterior parts of the body is carried to both 

 the kidneys and liver ; and in the carp, pike, 

 and perch, to the kidneys, liver, and vena 

 cava at once. The blood from the testicle, 

 ovary, swimming bladder, and kidneys, most 

 frequently goes to the vena cava.* 



Course of the blood in Invertebrate Animals. 

 In investigating the course of the blood in 

 animals destitute of a vertebral column and 

 cerebro-spinal nervous system, we are no longer 

 guided by any such analogies of form, posi- 

 tion, and use, as those just attempted to be 

 traced in the circulatory organs of the Ver- 

 tebrata ; for each class of Invertebrate animals, 

 as Mollusca, Articulata, and Zoophyta, and 

 even their subordinate orders, differ so widely 

 from one another in their organization, that 

 we are at a loss to discover any general plan 

 or type to which their circulatory organs may 

 be referred. 



In all of the Invertebrate animals in which 

 there is a regular progressive motion of the 

 nutritive fluids, there exists also a central con- 

 tractile organ to which the name of heart is 

 applied, from its functional rather than struc- 

 tural analogy to the central propelling organ 

 of the circulation in Vertebrate animals; and 

 in many of them, the outgoing and returning 

 vessels in which the circulation is performed 

 may be distinguished into arteries and veins, 

 by a difference of structure as well as of office. 

 From the same kind of analogy, the name of 

 auricle is given to the weaker part of the heart 

 of Invertebrate animals, which serves to re- 

 ceive the returning blood from the veins, when 

 such a cavity exists, and we call ventricle the 

 stronger and more muscular part which propels 

 the blood into the arteries. The general form 

 of these parts, however, and their position 

 relatively to the other systems, render it ex- 

 tremely difficult, if not altogether impossible, 

 to trace any strict anatomical correspondence 

 between the heart and bloodvessels of Verte- 

 brate and Invertebrate animals. In the Inver- 

 tebrate animals, the heart and principal artery 

 are generally placed on the upper part of the 

 body, above the alimentary canal and largest 

 portions of the nervous system ; while in all 

 Vertebrate animals the order is reversed, the 

 brain and spinal marrow being above, the 

 heart below the alimentary canal. 



In the Invertebrata, as in the higher animals, 

 the respiratory change of the blood is the most 

 important function to which its course or cir- 

 culation bears a constant relation. In the 

 Vertebrata the blood flows from the heart to 



* See the papers of Jacobson and Nicolai al- 

 ready referred to, and the extended Researches of 

 Rathke, Meckel's Archiv, 1826, and Aunal. des 

 Sciences Nat. torn. ix. 



the respiratory organ, while in the Invertebrata 

 the blood very generally arrives at the heart 

 after having passed through the respiratory 

 organ, and is propelled from the heart into the 

 systemic circulation : the vessels, therefore, in 

 which respiration is effected in the lower ani- 

 mals may be considered as belonging in ge- 

 neral to the venous circulation only, while in 

 the higher classes of animals, arteries alone, 

 or arteries and veins together, conduct the 

 blood through the respiratory organ. Another 

 remarkable difference between the circulation 

 of the nutritive fluids in Vertebrated animals 

 and that in the Invertebrate classes consists in 

 this, that in the first the digested food or chyle 

 and the lymph are taken up by a system of 

 vessels distinct from those circulating blood, 

 and are poured into the venous circulation 

 at one or more determinate places ; while in 

 the latter animals, the bloodvessels, so far at 

 least as we yet know, perform the office of 

 lacteal and lymphatic absorbent vessels as well 

 as of circulatory organs. In the Invertebrate 

 animals also, there is no vena portae, as in the 

 Vertebrata, and the liver is supplied with blood 

 only by a hepatic artery. 



In investigating the structure of the circu- 

 latory organs in different classes of Inverte- 

 brate animals, we at once perceive that no 

 accurate correspondence can be traced between 

 the varieties of their forms and the places 

 assigned to the animals in a Zoological arrange- 

 ment ; for we find among the Mollusca some 

 tribes having a highly developed and compli- 

 cated circulatory apparatus, and others with 

 heart and bloodvessels comparatively simply 

 organized. The same discrepancy occurs 

 among the Crustacea, Annelida, and Insects ; 

 and among the Entozoa and some other tribes 

 of Zoophytes, while some possess a simple 

 circulatory apparatus, in others we are not able 

 to discover any vestige of a vascular system. 



There is a considerable number of the lower 

 animals in which no vascular system has yet 

 been discovered, and in which the nutritious 

 juices are supposed to pass from the alimentary 

 cavity by interstitial transudation through all the 

 parts of their bodies. The circulation has, how- 

 ever, been recently shewn to exist in animals 

 formerly believed to be without it, and the 

 farther progress of Comparative Anatomy may 

 diminish still more the number of animals 

 believed to be destitute of circulating organs : 

 in the present state of our knowledge, it is 

 therefore as difficult to say with certainty in 

 what animals this function is deficient, as it is 

 to fix in which it is of the most simple or most 

 complicated kind. 



Mollusca. The greater number of the Mol- 

 lusca live in water and breathe by means of 

 gills, but many aquatic Mollusca, possessing 

 a branchial apparatus, appear to have their 

 blood aerated in other parts of the body also. 

 There is a strong muscular heart in all the ani- 

 mals belonging to this class, which when single 

 is always systemic, (figs. 320, 321, and 322, Jf.) 

 In the Cephalopoda, besides the aortic or sys- 

 temic heart, which has only one cavity or 

 ventricle, each vessel (fig. 320, B) leading to 



